No need of a Firefox add on. Just right click any bookmarks select properties and add keyword there. Now you can open your favourite site by just entering the keyword in the address bar. This post will help

Open Firefox and go to Extras > Edit Shortcuts (make sure to select 'Keyconfig' in the selectbox above, if you don't, you would edit the shortcuts just for a particular site)

Assign the shortcut of your choice and save.

Restart. You're done.

Comment: I know that some would point out that instead of editing the prefs.js directly you better should add an user.js in the mentioned directory. But when I customized the user.js, I had the problem that after every restart, the shortcut needed to be reassigned. So I ended up editing the prefs.js directly (which shouldn't be a problem anyway).